As well as having a tailored functionality, modern plastics are also intended to do increased justice to environmental concerns. As well as by a general optimization of preparation processes, this can also be achieved through the use of greenhouse gases, such as carbon dioxide, as building block for the synthesis of polymers. Accordingly, for example, a better environmental balance for the process can be obtained overall via the fixing of carbon dioxide. This path is being followed in the area of the production of polyether carbonates, and has been a topic of intense research for more than 40 years (e.g., Inoue et al., Copolymerization of Carbon Dioxide and Alkylenoxide with Organometallic Compounds; Die Makromolekulare Chemie 130, 210-220, 1969). In one possible preparation variant, polyether carbonate polyols are obtained by a catalytic reaction of alkylene oxides and carbon dioxide in the presence of H-functional starter compounds (“starters”). A general reaction equation for this is:

A further product, in this case an unwanted by-product, arising alongside the polyether carbonate polyol is a cyclic carbonate (for example, for R=CH3, propylene carbonate).
The literature describes a number of different preparation variants. For example, US 2010/0048935 A1 describes a process for preparing polyether carbonate polyols by reacting alkylene oxides and carbon dioxide with H-functional starter compounds by means of a DMC catalyst, in which one or more starter compounds are initially charged in a reactor and, in addition, one or more starter compounds are metered in continuously over the course of the reaction. One possible alkylene oxide mentioned is epoxidized soya oil. However, the reactivity of these oxirane rings is low, since they are within a chain and are highly sterically shielded. Therefore, the epoxidized soya oil is converted more slowly than standard monomers, such as propylene oxide, and accumulates in the reaction mixture. Since epoxidized soya oil, moreover, is a mixture of polyepoxidized compounds, controlled construction of defined polymer architectures is impossible.
WO 2006/103213 A1, in contrast, describes a process for preparing polyether carbonate polyols that features improved incorporation of CO2 into the polyether carbonate polyol, using a multimetal cyanide catalyst. The document discloses the presence of an H-functional starter, an alkylene oxide, and carbon dioxide in the presence of the multimetal cyanide catalyst in a reactor. The document further discloses the presence of a CO2-philic substance or of CO2-philic substituents. The CO2-philic substance or the CO2-philic substituent is intended to facilitate the incorporation of CO2 into the polyether carbonate polyol and so to reduce the formation of cyclic alkylene carbonates, such as propylene carbonate, for example, which represent unwanted by-products. The CO2-philic substance has to be removed from the reaction mixture obtained after the reaction, which leads to increased time demands and costs.
WO 2012/130760 A1 mentions the use of higher-functionality alcohols as starter in the reaction of epoxides with CO2 to give polyether carbonates under double metal cyanide (DMC) catalysis.
EP 0 222 453 A2 discloses a process for preparing polycarbonates from alkylene oxides and carbon dioxide using a catalyst system composed of DMC catalyst and a cocatalyst such as zinc sulfate. This polymerization is initiated here by one-off contacting of a portion of the alkylene oxide with the catalyst system. Only thereafter are the remaining amount of alkylene oxide and the carbon dioxide metered in simultaneously. The amount of 60% by weight of alkylene oxide relative to the H-functional starter compound, as specified in EP 0 222 453 A2 for the activation step in examples 1 to 7, is high and has the disadvantage that this constitutes a safety risk for industrial scale applications because of the high exothermicity of the homopolymerization of alkylene oxides.
WO 2003/029325 A1 discloses a process for preparing high molecular weight aliphatic polyether carbonate polyols (weight-average molecular weight greater than 30 000 g/mol), in which a catalyst from the group consisting of zinc carboxylate and multimetal cyanide compound is used, this catalyst being anhydrous and first being contacted with at least a portion of the carbon dioxide before the alkylene oxide is added. Final CO2 pressures of up to 150 bar place very high demands on the reactor and on safety. Even the extremely high pressure of 150 bar results in incorporation of only about 33% by weight of CO2 up to a maximum of 42% by weight of CO2. The accompanying examples describe the use of a solvent (toluene) which has to be removed again by thermal means after the reaction, thus resulting in increased time and cost demands. Furthermore, the polymers, with a polydispersity of 2.7 or more, have a very broad molar mass distribution.
For the conversion of the epoxide/CO2 copolymerization to the industrial scale, there is a need for a method for performing the reaction in a continuously operated plant that does not have disadvantages of the prior art, for example a long activation time and a significantly lower catalyst activity in the presence of CO2 and short-chain starter compounds and, following from this, a low mean reaction rate and a long residence time of the reaction mixture in the reactor (corresponding to a low specific product output).
A high specific product output is understood in the context of the invention to mean a high mean reaction rate of the addition of epoxides and CO2 onto H-functional starter compounds (“copolymerization”). The specific product output of a reactor or, more generally, of a process can be determined as the quotient of the mass flow rate of alkylene oxide used and starter compound multiplied by the conversion of alkylene oxide obtained and the volume of the liquid phase in the reactor in question. The specific product output is reported in kg/(m3·h).
The problem addressed was therefore that of providing a process for preparing polyether carbonate polyols which features a high specific product output, and wherein the resulting polyether carbonate polyol has a narrow molar mass distribution (low polydispersity) and a minimum viscosity.